With good electrical insulating properties, heat resistance, and mechanical characteristics, polyimides have been used in a variety of fields including semiconductor production. Polyimides generally tend to be insoluble in solvents and thermally infusible and accordingly, difficult to mold or process directly. For film formation, therefore, a solution (hereinafter, referred to as varnish) containing polyamic acid as a precursor of a polyimide is commonly used to form polyimide film through coating and curing steps for conversion. Such a varnish may be a solution as obtained from polymerization of polyamic acid or may be prepared by dissolving polyamic acid in a solvent.
In general, mechanical characteristics (elongation percentage and ultimate stress) of polyimide film can be improved effectively by increasing the degree of polymerization of the polyimide. As the degree of polymerization of polyamic acid increases, however, the viscosity of the polymerization solution increases, often causing troubles in the polymerization process. In addition, it will be difficult to adjust the varnish to a viscosity suitable for coating. The viscosity of a varnish can be adjusted appropriately through control of the polymerization degree of polyamic acid by changing the molar ratio between the acid anhydride group in the acid dianhydride monomer used and the amino group in the multivalent amine compound or diamine compound during the polymerization of polyamic acid. However, polyimides produced from this varnish have the same polymerization degree as the polymerization degree of the original polyamic acid, making it impossible to achieve high mechanical characteristics.
Accordingly, methods in which the polymerization degree is controlled by adding water or alcohol during the polymerization of polyamic acid to cap the acid anhydride end (Patent documents 1 and 2) and methods in which terminal amino groups are capped to control the polymerization degree of polyamic acid (Patent document 3) have been reported. If these methods are used, the end capping agent will be removed during the curing step, allowing the acid anhydride group and amino group to regenerate and take part in the polymerization reaction again. As a result, the polymerization degree of the resulting polyimide will increase, making it possible to produce polyimide film with good mechanical characteristics.
Besides the above ones, methods in which an end capping agent containing a thermally polymerizable group is used to introduce a thermally polymerizable group into the polyamic acid end have been reported (Non-patent documents 1 to 4). These methods are intended to improve the mechanical characteristics of polyimide film by causing reaction between the thermally polymerizable end groups during the curing step.